1. Field of the Invention
The present invention relates to a high-voltage ceramic capacitor on which a high voltage is applied and a method of manufacturing it.
2. Description of Background Art
Generally, dielectric ceramic such as barium titanate has a high dielectric constant and a small tan .delta. and, therefore, a capactor is obtainable that has a comparatively large capacity and high withstand voltage despite a small size. For this reason, capacitors using such a material have been utilized so far also for electric power equipment, for example, for surge-absorbing capacitors for circuit breakers.
FIG. 3 and FIG. 4 are cross-sectional views respectively showing one example of conventional capacitors for electric power equipment.
As shown in FIG. 3, a capacitor 1 comprises a cylindrical dielectric ceramic element 2, and a silver electrode 3 is formed on the top surface thereof and a silver electrode 4 is formed on the bottom surface thereof, respectively. The capacitor 1 is formed so that the diameters of the electrodes 3 and 4 are somewhat smaller than the diameter of the element 2, and thereby the surface distance between the ends of the electrodes 3 and 4 is elongated. This improves the pressure-resisting characteristic of the capacitor 1 against creeping discharge. On the center portion of the electrode 3, an external terminal 5 is bonded through a conductive adhesive 6. Similarly, on the center portion of the electrode 4, an external terminal 7 is bonded through a conductive adhesive 8. Furthermore, the is surrounded by a molding comprising dielectric ceramic element 2 are molded with an insulating resin 9. Female threads are formed in the external terminals 5 and 7 to allow leads to be connected or to allow other capacitors of the same shape to be connected in series.
Also, the capacitor as shown in FIG. 4 has nearly the same structure as that of the capacitor as shown in FIG. 3, but differs in that aluminum is used for the material of the electrodes 10 and 11 and glasses 12 and 13, the main component of which is borosilicate lead glass, are fusion-stuck in an annular shape on the end-edges of the electrodes. Formation of glass portions on the end-edge portion of the electrodes 10 and 11 in such a manner can improve the pressure-resisting characteristic against creeping discharge. Also, the glass portions 12 and 13 have a comparatively higher dielectric constant than that of the insulating resin 9, and therefore electric field concentration at the electrode end is alleviated, resulting in an improvement in the breakdown voltage of the dielectric ceramic element in the vicinity of the electrode end part. However, the above-described conventional high-voltage ceramic capacitor may suffer problems, as noted below.
Where the breakdown voltage of capacitors is measured, two methods are employed: a method wherein high-voltage impulses are applied across the external terminals of a capacitor to be measured and a breakdown test is conducted by gradually raising the voltage thereof; and a method wherein a large number of capacitors to be measured are connected in a series-parallel fashion, high-voltage impulses are applied across two terminals of the capacitor circuit, and a breakdown test is conducted by gradually raising the voltage thereof. However, in some cases, the value of breakdown voltage per one capacitor obtained by the method of connecting a large number of capacitors in a series-parallel fashion was lower than the value of breakdown voltage obtained by the method of measuring a single capacitor. This phenomenon took place where a specific conductive adhesive is used for, bonding between the electrode and the external terminal, and remarkably did not appear where a conductive adhesive was used which was prepared with a special component and a special combining ratio.
A possible cause of a reduction in the value of breakdown voltage per one capacitor where a large number of capacitors are connected in a series-parallel fashion to be measured is that particularly when an impulse voltage having a very short duration of wave front is applied, the resistance value attributed to the resin component in the conductive, adhesive is increased sharply; and a high voltage is thus applied to the conductive adhesive portion between the electrode and the external terminal, and an imbalance of voltage share of each part of the capacitor takes place. In such a case, when a breakdown occurs at the conductive adhesive portion, the voltage share of each part of one capacitor and the voltage share of each of a plurality of capacitors change sharply, and a capacitor to which the highest voltage is applied can conceivably be broken down.
The above-described problem does not take place if the external terminal can be bonded to the electrode by soldering, but in a capacitor having a high breakdown voltage as shown in FIG. 4, a base metal such as aluminum, was used for the electrode, and therefore soldering could not be applied. In addition, it is also considered to apply glass treatment as shown in FIG. 4 to the capacitor of a silver-electrode type as shown in FIG. 3, but this hardly improves the impulse voltage-resisting characteristic in comparison with the case of applying glass treatment to the base metal electrode.